Of ‘the coexistence of closely similar structures of diverse origin,’ illustrated so forcibly by the instance of the eye, ‘in at least three independent lines of descent, the Mollusca, the Annulosa, and the Vertebrata,’ it can scarcely be denied that Natural Selection alone would be an inadequate explanation. But here again it should be observed that Darwinism does not attribute everything to Natural Selection. It assumes, what must be allowed, that variations occur. In obedience to what laws those variations themselves are produced is an interesting speculation, and a most important subject of inquiry. That such laws or conditions of Variation exist no one can doubt, unless he has been seduced by Ovidian metamorphoses to believe in trees bleeding human blood and human foreheads branching with the antlers of the stag. A knowledge of those conditions might fully explain the coexistence of similar structures of diverse origin, consistently with the principle of Natural Selection. The ignorance of them is scarcely a proof that such coexistence does not harmonize with it. The objection that giraffes, which profited by long necks in a time of drought, would find them a disadvantage subsequently, as requiring a greatly increased size and strength of muscles to support them, overlooks the law of correlation, by assuming that the elongated neck would be out of proportion to the other conditions of the creature’s fabric. Mr. Mivart’s fourth objection seems at least an extremely improbable opinion. He refers to Mr. Darwin’s expression, that the goose appears to have a highly inflexible organization, as if he himself thought it possible for a species at length to attain to an organization completely inflexible. Such a view would imply two parents exactly like one another, producing offspring exactly like themselves; and of such exact likenesses no known families afford examples. The seventh objection recalls the still unexplained physiological difference between ‘species’ and ‘races,’ unions between the former being sterile, and between the latter fertile. In this branch of the subject there is much scope still for inquiry. Some of the difficulty may be due to a trick played us by language. True species have been defined to be those that are not fertile together; and from the definition it follows that races which _are_ fertile together are not true species. But the question is obscured by the use of the two different words ‘races’ and ‘species,’ the real issue being, whether races that are and races that are not fertile together can originate in the same way. The subject in its other bearings has been largely discussed by Mr. Darwin in his work on ‘Animals and Plants under Domestication.’